Beverage dispensing machines are often equipped with a liquid container, e.g. a water tank that is usually located on the backside of the machine. Those liquid containers contain a defined amount of liquid with which, depending on a chosen recipe, a couple of drinks, e.g. coffee drinks, can be made. Often these liquid containers are equipped with a minimum level sensor to avoid that the pump, which is used for extracting the liquid from the liquid container, starts to run dry. These minimum level sensors usually also send a signal to the user indicating that the liquid container is empty and needs to be refilled.
Nowadays coffee machines often use minimum level sensors that comprise a float with a magnet that relates to a Hall sensor. The Hall sensor is located at a minimum level of the water tank, so that the device can signal if the recipe, i.e. the type of coffee selected by the user, may be produced with the water content available in the water tank.
The problem with the aforementioned minimum level sensors is that they are not very precise. Also these sensors are usually located at a minimum liquid level that relates to the volume of the largest drink/recipe that is possible with the machine. In some cases it might thus happen that a user selects only a small drink (e.g. an espresso), wherein the appliance indicates that the water tank needs to be refilled, even though this would actually not be necessary. If the minimum liquid level is, for example, set to 180 ml, the system would not allow brewing any coffee or espresso, even if the recipe ordered by the user requires only 80 ml.
Since the water tanks are usually located on the backside of the machines, it is also difficult for the user to visually judge the water content of the tank. Thus, there is a need for measuring the volume of the water tank in a precise way and allowing the machine to brew a coffee or espresso if the available water content is suitable to brew the kind of coffee or espresso selected by the user.
Conventional manual coffee machines often use a manual scale reading which is arranged at a riser pipe that is fluidly connected to the water tank. Such a system is exemplarily known from EP 1 514 500 A1. The therein shown system comprises a main tank and a riser pipe which are fluidly connected to each other as communicating vessels. The riser pipe contains a float by means of which the user may visually identify the water level within the main tank. Such a system, however, only helps to indicate the relative water level within the main tank, but does not solve the above-mentioned problems that the user should be informed whether the remaining water content in the main tank is sufficient for the recipe he/she has chosen and that it should be electronically prevented that the main tank gets empty during coffee brewing such that the pump is running dry.
Of course, there are also more sophisticated measurement devices and sensors available in the market that meet the above-mentioned requirements and allow measuring the volume of water within a water tank in a very precise manner. However, these measurement devices and sensors most of the times require a rather expensive equipment which would disproportionally increase the manufacturing costs.
WO 98/27853 A1 discloses a coffee maker comprising a water reservoir, a water heating and supply device for supplying hot water to a filter device, the brewed coffee flowing from the filter device to a receptacle. The coffee maker includes a level detection circuit for detecting the water level in the reservoir, and this water level is used to control the brewing process so as to vary the flow of hot water to the filter device in dependence upon the water level in the reservoir. The water level may be used to control the power of a heating element in a continuous heater or may be used to control the operation of a pump in a pumped system.
US 2004/118292 A1 discloses an appliance for the preparation of hot drinks. The appliance includes a water container having a container outlet from which, via a heating device and a riser, water heated by the heating device can be supplied to a mixing device. A specific water level height in the riser can be kept at least largely constant by a constant level holding device.
GB 2 320 093 A discloses a method for totalizing the volume of liquid pumped to or from a vessel by a set of pumps.